staging: comedi: serial2002: Use monotonic clock

Wall time obtained from do_gettimeofday is susceptible to sudden jumps due to
user setting the time or due to NTP.

Monotonic time is constantly increasing time better suited for comparing two
timestamps.

Signed-off-by: Abhilash Jindal <klock.android@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>